Record android-arm64 zig platform in mise.lock (#16)

mise widened the lockfile when resolving zig on the Termux/Android
dev host. The entry mirrors linux-arm64 (same checksum/URL — the
zig-aarch64-linux tarball), keeping test:fast reproducible there.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: gtbuchanan